Smaller electronic, optical and micromechanical devices, with nano-scale device manufacturing already on the horizon, are driving the need for improved cleaning and drying technology. Smaller circuits and surface features are becoming increasingly affected by smaller surface particles and other residues (contaminations) during manufacturing operations. Moreover, many conventional wet cleaning techniques are not compatible with the shrinking device geometries and new manufacturing materials. Still moreover, the transition to larger wafer substrates such as the 300 mm platform is driving the need for increased performance and productivity in cleaning and drying processes and tools. Precision cleaning and drying are common and often repeated process steps performed prior to or following almost every other non-cleaning step of wafer manufacturing, for example photoresist deposition, curing, acid etching, plasma etching and patterning. As such, a present need exists for novel substrate dry cleaning and drying technology, as well as the capability of integrating new dry cleaning and drying technology with the aforementioned conventional wafer manufacturing processes, wet or dry, to provide a more efficient and integrated wafer processing tool.
Much interest exists to develop alternative wafer cleaning, drying and photoresist deposition methods to replace hazardous cleaning and drying chemicals such as alcohols, hydroxylamines, and organic solvents, or to produce surfaces, films or coatings with increased quality, planarity and thinness. The presence of organic contaminants or particles on a substrate surface with thicknesses on the order of 0.1 microns or greater generate considerable cleaning difficulties. Often it is necessary to remove all gross contamination first by solvent immersion cleaning before applying precision spray cleaning procedures. In another example, vacuum plasma cleaning using argon-oxygen or fluorine chemistry is used to treat a layer of patterned photoresist. Often this procedure leaves behind trace amounts of recalcitrant reactive ion etched (RIE) polymeric residues comprising a mixture of metals, carbon and fluorine. A second precision cleaning step using acids, alkalies, peroxides, ozonated water, or hydroxylamine chemistries, followed by deionized water rinses and alcohol drying are required to produce precision clean and dry surfaces.
The trend towards miniaturization of silicon, germanium and gallium arsenide microprocessors in the electronics industry and the emergence of new micro-electromechanical systems (MEMS) manufacturing, which uses much the same microprocessor manufacturing technology, is creating new material and process challenges. Conventional cleaning, drying, etching, and deposition technologies are being pushed to their limits as contamination removal issues become more important with each new device generation. For example, the smaller dimensions create new cleaning challenges due to increasing capillary force pressures which hold process fluids within cavities, more prevalent electrostatic forces which hold micromechanical structures together, porous or complex surface topography which preclude the use of aqueous chemistries, and high aspect ratio cavities and vias which hide etch residues and particles, among others. Moreover, new materials such as low-k films and copper lines used to fabricate smaller device geometries (line widths) are not compatible with many conventional wet processing techniques described above.
For example, surface micromachining of poly-silicon films deposited on silicon wafers is an emerging technology in the fabrication of micro-actuators and micro-sensors. These miniaturized components include micro-engines, micro-lever actuators, accelerometers, optical switches, biomedical sensors, and pressure sensors which have potential uses in a variety of applications for mechanical and electrical devices both in industry and in government.
Similar to integrated circuits (IC) fabrication, surface machined microstructures are formed using a combination of masking, dry plasma etching of poly-silicon film deposited on the wafer, and wet etching done in a liquid-phase acid solution such as hydrofluoric acid (HF). A final HF etch is followed by a water rinse. In some cases the HF etch is followed by an ammonium fluoride (NH4F) treatment. After etching with acid and rinsing with deionized water, the part is dried, for example using evaporative methyl alcohol drying, to yield the released micromachined sample.
Silicon is a very practical micromechanical material in that it is capable of a great amount of flexibility before fracturing. However, the compliant nature of the silicon makes it susceptible to fabrication problems. A significant problem in the fabrication of the micromachined components is sticking of released structures to the substrate after they are dried using conventional air drying techniques. The sticking, combined with static friction which these parts experience has been termed stiction, a phenomenon commonly seen in magnetic storage media. A number of phenomena may potentially cause microdynamic stiction of suspended microstructures, several of which will be identified here. Electrostatic forces due to electrostatic charging may cause sticking. These forces can be generated on the wafer due to etching, rinsing and drying operations. This is a non-equilibrium condition which usually dissipates over time or with contact between conducting surfaces. Second, a smooth surface finish may cause stiction. Smooth surfaces are more likely to stick, while surface roughness effectively increases the nominal separation between micromachined surfaces. Third, a phenomenon called solid bridging occurs when non-volatile impurities present in the drying liquid are deposited on the surfaces of the microstructures. The impurities in narrow gaps formed by the suspended microstructures essentially bridge the gaps, causing the structures to stick. Obviously, avoiding impurities in the liquid cleaning and drying fluids will minimize solid bridging.
Perhaps the most troublesome cause of surface stiction is liquid bridging. Liquid bridging is due to the surface tension effects of trapped capillary liquids upon drying. The liquid, usually water, used to rinse the microstructures is trapped in the narrow gaps between the silicon wafer and the suspended structures. Interfacial forces generated when the trapped capillary fluid dries can cause the microstructures to collapse and stick. Moreover, conventional thermal or solvent drying of silicon IC structures such as microvias cause the cavity walls to crack as the sidewalls are pulled together during extraction or evaporation of water or high surface tension drying solvents such as methyl alcohol.
The meniscus force (Fm) between two flat, polished surfaces (or wafer microvia sidewalls) with a liquid bridge is given by LaPlace's law and is calculated using the following equation;Fm=(γA/h)(cos φ1+cos φ2), where;                φ1—contact angles of the liquid with the surface 1;        φ2—contact angles of the liquid with the surface 2;        A—shared surface area of the two parallel surfaces, assuming the gap between them is flooded with capillary liquid;        h—average thickness of the liquid bridge;        γ—surface tension, 73 dynes/cm for water at 25 C.        
If a liquid such as water is present in small capillaries during the drying process, the surface tension exerts tremendous pressure on the sidewalls. This stress can be high enough to cause smoothe flat interfaces to stick, or in the case of IC fabrication, microvia sidewalls to collapse. Thus it is very beneficial to reduce or eliminate surface tension to lessen or eliminate surface stiction due to liquid bridging and prevent sidewall collapse of very small trenches during drying.
Supercritical carbon dioxide (SCCO2) extraction has been demonstrated to remove capillary liquids (e.g. methanol) from micromachined structures, eliminating sticking caused by surface tension effects. Carbon dioxide has long been known to be a good solvent for many organic compounds and methanol, in particular, is known to be very soluble in SCCO2. A conventional wafer drying process using carbon dioxide has been to displace the rinse water first with methyl alcohol, and then to dissolve the methanol with liquid or supercritical carbon dioxide. If extremely small capillaries are present, the process may be performed above the critical temperature of methanol, which can be over 200 C, to insure that the interfacial surface tension of the drying and extraction fluids within these small capillaries is approximately 0 dynes/cm. After the methanol has been dissolved and carried away by the supercritical fluid, the vessel is depressurized to yield dry, released microstructures or dry vias and trenches. Surface tension effects are eliminated since SCCO2 has negligible surface tension like a gas. Furthermore, SCCO2 exhibits gas-like properties of diffusivity and viscosity which allow the supercritical fluid to access narrow gaps under the microfeatures for removal of trapped capillary fluid. Other commercial uses of dense fluids for drying include the extraction of solvents from phase-separated polymer gels to produce microcellular foams and the extraction of solvents from silica aerogels. Also, researchers at the University of California at Berkeley have removed drying solvent from micromachined samples using liquid carbon dioxide which requires the additional step of increasing to supercritical conditions before depressurizing to avoid the sticking problems caused by a liquid/vapor interface.
The most significant drawbacks with the aforementioned conventional dense fluid drying techniques are very long process cycle times and the use of excessive amounts of supercritical or liquid carbon dioxide in completely flooded pressure vessels to remove only trace amounts of surface contamination (i.e., water and drying solvents). Another drawback is that these drying methods do not effectively remove small particles and in fact can easily re-contaminate substrates which are completely bathed in the reactor fluid. Moreover, these methods are not effective or selective for removing other liquid contaminations present on the substrate surface or trapped within pores of substrates. Still moreover, solid contaminants such as carbon residues are not effectively removed using these conventional techniques, even when modified with organic solvents. Most often extreme pressures are required to achieve separation.
Water, solid particles and residues (contaminations) must be removed from critical surfaces. If left on critical surfaces, these may bridge circuits, obscure light or produce other deleterious side-effects which reduce yields, that is clean dry surfaces for subsequent processing steps. For example, surface residues, particles and liquids such as water must be removed prior to placement or in-situ formation of thin coatings such as low k dielectrics, or patterning, in preparation for subsequent lithographic processes. Moreover, processes described above such as cleaning, etching, drying and application of coatings are most often performed as separate operations, which greatly increases the risk of device contamination during manufacture.
With respect to cleaning wafers to remove trace organic and particle contamination, commercial wet and dry cleaning systems have been developed which employ ozone and water to replace dangerous or ecologically-unsafe chemical processes such as sulfuric acid-hydrogen peroxide mixtures, toxic organic solvents, and amine-based cleaning agents. One such system, called the SMS DIO3 photoresist strip process (Legacy Systems Inc., Fremont, Calif.), uses an ozone generator and diffuser located in a tank of chilled (5 C) deionized water which is circulated into a tank containing the wafers. This system suffers from an inability to apply thermal energy to the substrate because it lowers the solubility of ozone in solution and is essentially time-dependent and concentration-dependent solid-ozone gas interfacial reaction. Another commercialized process, called HydrOzone (trademark of Semitool Inc.), diffuses ozone gas through a thin film of heated water which is spreading over a spinning wafer. The HydrOzone process is claimed to be more efficient than the DIO3 process above because the water component may be heated to provide thermal cleaning energy and RPM may be varied to control boundary layer thickness. However, similar to the DIO3 process, transport of ozone of any significant concentration into micron features on the wafer surface is very limited due to the solid-ozone gas interface. Moreover excessive agitation caused by rapid movement of water over the spinning wafer accelerates the decomposition of the ozone gas as it diffuses through the thin film boundary. Moreover, complete drying of the substrate following cleaning by both methods is also limited due to hydration of small capillaries, vias and interstices present on the wafer. Finally, a lack of solvent selectivity can be limiting in many resist removal applications. Commercial cleaning of textiles using ozonated water is also known. Ozone acting as a cleaning agent additive is used to destroy soils contained on fabrics. This method is similar to ozonated water treatment of wafer and suffers from the same solubility and selectivity problems.
Following these ozonated processes, a technique often used to rinse wafers is the “quick dump” method. The quick dump method relies upon the rapid deployment of water from the rinse tank to remove water and impurities from the semiconductor wafer. A limitation with this method is its inability to actually remove particles from the wafer. In fact, the rapid deployment of water from the tank often transfers more particles onto the wafer. In addition, the wafers from the quick dump tank must still undergo a drying operation, further increasing the number of particles on the wafer. As previously noted, an increase in particles on a surface often relates to lower die yields on the semiconductor wafer.
A further technique used to both rinse and dry wafers relies upon a spin rinse/dryer. The spin rinse/dryer uses a combination of rinse water spray to rinse and centrifugal force to remove water from the semiconductor wafer. The dry step removes water from the semiconductor wafer substantially by centrifugal force and evaporation. However, the spin rinse/dryer often introduces more particles onto the wafer. In fact, initially dissolved or suspended contaminants such as particles in the water are often left on the semiconductor wafer, thereby reducing the number of good dies on the wafer. Another limitation with the spin rinse/dryer is its complex mechanical design with moving parts and the like. The complex mechanical design often leads to problems such as greater downtime, wafer breakage, more spare parts, and increased cost of ownership, among other issues. A further limitation is static electricity often builds up on the wafers during the spin cycle, thereby attracting even more particles onto the surface of the semiconductor. Accordingly, the spin rinse/drying does not clean or remove particles from the wafer.
Other techniques used to dry wafers include an isopropyl alcohol (IPA) vapor dryer, full displacement IPA dryer, and others. These IPA-type dryers often rely upon a large quantity of a solvent such as isopropyl alcohol and other volatile organic liquids to facilitate drying of the semiconductor wafer. An example of such a technique is described in U.S. Pat. No. 4,911,761, issued to McConnell et al., which generally suggests the use of a superheated or saturated drying vapor as a drying fluid. This superheated or saturated drying vapor often requires the use of large quantities of a hot volatile organic material. The superheated or saturated drying vapor forms a thick organic vapor layer overlying the rinse water to displace (e.g., plug flow) such rinse water with the drying vapor. The thick vapor layer forms an azeotropic mixture with water, which will condense on wafer surfaces, and will then evaporate to dry the wafer.
A limitation with this type of dryer is its use of the large solvent quantity, which is hot, highly flammable, and extremely hazardous to health and the environment. Another limitation with such a dryer is its cost, which is often quite expensive. In fact, this dryer needs a vaporizer and condenser to handle the large quantities of hot volatile organic material.
Still another technique relies upon a hot deionized (DI) process water to rinse and promote drying of the semiconductor wafer. By way of the hot DI water, the liquid on the wafer evaporates faster and more efficiently than standard room temperature DI water. However, hot water often produces stains on the wafer, and also promotes build-up of bacterial and other particles.
U.S. Pat. No. 6,240,936, issued to DeSimone et al., suggests a method for applying liquid carbon dioxide onto a portion of spinning substrate to clean and deposit solutes contained within the liquid carbon dioxide spray agent in a carbon dioxide atmosphere. The '936 patent suggests using inert gases with liquid carbon dioxide and a pool of liquid carbon dioxide within the process chamber, both methods being taught to maintain a differential pressure for selectively evaporating the liquid carbon dioxide from the substrate surface. However, the '936 patent does not teach first establishing an inert supercritical fluid atmosphere into which a liquefied gas or supercritical fluid such as carbon dioxide may be much more selectively applied and controlled. Methods as suggested by the '936 patent suffer from an inability to fully exploit the delivery agent (dense fluid) chemistry, as the liquid-state carbon dioxide chemistry is not a variable geometry dense fluid. The application temperature must be maintained below 30 C to maintain liquid phase and solvent power. This is a significant disadvantage as elevated temperatures improve spray cleaning performance, lowering particle adhesion and increasing contaminant solubility, and increases the solubility of high molecular weight polymers such as photoresist resins. For example, the liquid phase carbon dioxide surface tension, density and viscosity in the '936 patent cannot be varied to any significant degree, which prevents optimization of solute chemistry and substrate cleaning and deposition processes. Moreover, the substrate is contained in the saturated dense fluid vapor atmosphere which requires large quantities of dense fluid and extra processing steps to remove. Still moreover, it has been the experience of the present inventor using the exemplary commercial SuperFuge cleaning system described above that using liquid carbon dioxide cleaning solvent at a pressure and temperature on the vapor-liquid equilibrium boundary produces an inferior surface cleanliness to liquid carbon dioxide compressed above its liquid-vapor boundary. However, to achieve this later state the entire pressure vessel—substrates, baskets, pressure walls, centrifuge assembly—must be in liquid phase. Using this method, most of the cleaning fluid is wasted and cross contamination problems arise.
By contrast, the present invention provides a means for producing variable geometry supersaturated liquids (liquids compressed above their normal liquid-vapor pressure-temperature boundaries) and supercritical fluids which are selectively contacted with a substrate surface. Using an anti-solvent atmosphere to first to create the proper temperature-pressure conditions within which a much small quantity of dense fluid is selectively introduced, used, and captured. The present invention is more selective, uses less reagent (dense fluids), and the techniques taught herein extend themselves to cleaning, surface modification and deposition processes.
In contrast to the '936 patent, the present invention teaches selectively applying preferably a supercritical fluid, but a liquefied gas may be applied, which may contain one or more substances, to a substrate surface which is either colder or hotter than the applied dense fluid. The substrate is made warmer or colder than the dense fluid using an inert supercritical fluid anti-solvent atmosphere which first bathes the substrate before selective application of the dense fluid. Still moreover, and in contrast again to the '936 patent, the substrate and reactor temperature of the present invention is controlled much more precisely during depressurization processes. This is accomplished because the joule-thompson (J-T) coefficient of the supercritical atmosphere is much lower than the J-T coefficient of the dense fluids, the volumetric ratio of supercritical atmosphere to dense fluids is very large, and the extraction of dense fluids during processing is performed in such a manner as to prevent excessive amounts of dense fluid from ‘boiling off’ on substrate surfaces.
U.S. Pat. No. 5,908,510, issued to McCullough et al., suggests a method for residue removal from etched wafers using supercritical or liquefied carbon dioxide, followed by a jet spray of solid cryogenic particles. The '510 patent suggests using surfactants and additives under a contact time of between 30 minutes to 2 hours to remove CFx residues from vias and trenches using stirred fluid at about 500 to 2500 rpm. The additives are water and a fluorinated surfactant. As discussed above in regard to the '936 patent, the '510 patent suffers from lack of selectivity, with the entire substrate processed in essentially carbon dioxide. The '510 patent does not teach creating reactive cleaning agents in-situ nor selectively applying said instantaneous cleaning agents to contaminated surfaces to achieve a desired substrate surface cleanliness in a much more rapid period of time. Moreover, in contrast with the '510 patent, the present invention is a much more effective surface cleaning (i.e., particles, residues) technique called condensation shear surface cleaning which itself is greatly enhanced using supercritical plasma techniques described herein.
U.S. Pat. No. 5,013,366, issued to Jackson et al., suggests stepping the temperature between the liquid state and supercritical state in a series of steps. The entire substrate is bathed in a dense fluid environment which is then changed physicochemically by changing the chemistry of the bulk fluid in a series of steps over time using bulk fluid temperature changes from above and below the critical temperature of the dense fluid. The process of the '366 patent is inefficient because it changes the properties of an entire fluid environment to process a substrate surface. Also the '366 patent will not remove small surface particles because it does not produce sufficient shear stress energy during the phase transition. By contrast, the present invention uses a much smaller fraction of dense fluid which is selectively applied to a substrate surface and is changed from supercritical fluid phase to liquid phase, or vice versa, in a single sweeping and radial step, and preferably in a pulsed application while traversing the surface of a substrate surface—producing both shear stress and a change in chemistry. As such and by contrast, the object of the present invention is to provide a shearing action across the surface of a substrate, as well as change the viscosity, cohesive energy and density of the fluid while it traverses from the center to the perimeter of a planar substrate.
U.S. Pat. No. 5,403,621, issued to Jackson et al., suggests non-selective deposition process of coating an entire substrate surface by stepping the temperature between the liquid state and supercritical state to cause a coating to drop out of solution. The entire substrate is bathed in a dense fluid environment which is then changed physicochemically by changing the chemistry of the bulk fluid in a series of steps over time using bulk fluid temperature changes from above and below the critical temperature of the dense fluid. The process of the '366 patent is inefficient because it changes the properties of an entire fluid environment to process a substrate surface, causing the coating to coat all surfaces of the reactor and substrate simultaneously. By contrast, the object of the present invention is to provide a selective method for producing varying thickness of a coating over a portion of a substrate surface while a dense fluid mixture traverses from the center to the perimeter of a planar substrate.
In all of the cited prior art, none suggests the use of a condensation shear cleaning technique whereby a condensable solvent phase is selectively contacted with a substrate surface or portion thereof to cause the solvent phase to “locally” condense due to heat exchange with the substrate surface, while the substrate is dynamic or static. Moreover, the prior art does not propose the novel and beneficial aspects of combining plasmas with dense fluids.
Another novel and beneficial aspect of the present invention is that a supercritical fluid anti-solvent such as nitrogen or argon provides enough vapor pressure to essentially condense and contain most of the carbon dioxide compound as a liquid in a hemisphere which is below the substrate. This allows for easy recovery and reuse of most of the carbon dioxide through headspace compression and minimizes joule-thompson cooling of the substrate being treated during post-processing depressurization operations. Because of a comparatively small ratio of dense fluid-to-supercritical atmosphere (i.e., approx. 5 parts in 100) a small quality of condensed dense fluid (liquid) is continuously discharged from the bottom of the process chamber and only inert supercritical atmosphere, having a much lower joule-thompson coefficient than the dense fluid cleaning/deposition agent, remains following each spray cleaning cycle.
An excellent description of a conventional coating process is the photoresist deposition process and is detailed in VLSI Fabrication Principles, Second Edition, Sorab K. Ghandi, summarized as follows. In a conventional process, a film of photoresist material is placed on a surface of the wafer which is covered by a masking film. It is desirable to have the photoresist film be very uniform, highly adherent, and completely free of fine particulate matter or pinholes. For example, a positive resist is applied using a suitable organic carrier solvent. A pre-filtered resist-solvent mixture is applied to the center point of a spinning wafer, whereupon the coating mixture spreads outward in all directions from the center to the perimeter of the wafer. The film thickness is inversely proportional to the square root of the spin rate; typically, spinning speeds range from 1000 to 6000 rpm and result in film thickness that ranges from 0.5 to 3.0 um. Consistent results are maintained only if the viscosity of the resist-solvent mixture is maintained on a run-to-run basis. Extreme care must be taken to clean and dry a wafer prior to photoresist deposition to prevent adhesion problems and incorporation of surface particles. Where surface adhesion is an issue due to, for example, the presence of certain functional groups, clean and dry wafers may be pre-wetted with a suitable organic coupling agent such as hexylmethyldisilizane (HMDS), trichlorophenylsilane, trichlorobenzene or xylene using a dipping or vapor-plating technique. Ultra-clean conditions must be maintained during the coating step to prevent the inclusion of atmospheric particles into the coating matrix, caused by air turbulence from the spinning wafer. The photoresist is sticky at this point and great care must be taken to prevent particle contamination. Following deposition, the coated wafer is soft-baked, preferably from below to remove excess organic solvent and vapors contained in the thin sticky film, without forming a “skin” on top. The bake temperature may be between 90 C to 100 C and causes the film to shrink to about 85% of its original thickness.
Following cleaning, coating and soft-bake procedures, the coated wafer is moved to a mask alignment, exposure with collimated UV light, post-bake, development, etching and stripping. Plasma cleaning processes are widely used to remove patterned photoresist, however radiation damage to MOS circuits can be a problem as well as an inability to remove carbon-fluorine sidewall contaminants produced by deep reactive ion etching (RIE) processes. These etch residues must be removed prior to repeating the coating, soft-bake and other lithographic processes described above. One aspect of the present invention teaches removing RIE residues from plasma treated substrates.
Following is a more detailed discussion of conventional substrate cleaning, preparation and coating techniques in relation to the present invention. This discussion will expose additional novel and beneficial aspects of the present invention.
High Pressure Solvent Spray Cleaning
Solvent spray cleaning provides an alternative process for removing particulate contaminants from flat surfaces. This technique effectively removes submicron particles proportionally with increasing spray pressures due to the production of high shear stress within the surface boundary layer where small particles hide out. To avoid recontamination of the surfaces, spray cleaning should be practiced in a closed environment or under a flow of clean inert atmosphere. Solvent spray cleaning techniques include high pressure alcohol and fluorocarbon solvent spray, steam sprays, dry ice pellet blasting and particle ice snow spray cleaning. These techniques are useful for cleaning 2-D surfaces only and cannot permeate of penetrate substrate pores, even at extremely high spray pressures due to access problems. However, techniques such as carbon dioxide snow cleaning, pellet cleaning, argon ice cleaning are excellent polishing techniques for removing sub-micron particles (particle cleans).
An aspect of the present invention is to optimize and control particle drag or shear stress as a means for cleaning and/or depositing a film on a planar substrate surface. Moreover, cohesion energy of the same fluid spray may be controlled to optimize penetration into a porous substrate surface to remove trapped contaminants therein. Another novel aspect of the present invention is to combine an atmospheric plasma process with a solid phase spray head to combine the benefits of atmospheric plasma cleaning with solid-phase dense fluid spray cleaning. A patented and patents-pending TIG-Snow carbon dioxide spray technique developed by the present inventor is used in an example final particle cleaning device and technique, however any dielectric spray technique described above such as argon ice, pellet carbon dioxide and liquid nitrogen cleaning will work with an atmospheric plasma to produce an ultraclean and particle free surface.
Plasma Cleaning
Plasma cleaning is a type of so called dry cleaning procedure which has the advantage of being able to clean large quantities of samples with little or no waste products and little operator input. It also has the distinct advantage of directly generating a clean, uniformly wettable and dry surface on the substrate. The cleaning may involve an argon-oxygen plasma to directly oxidize hydrocarbons, or an argon-only plasma to degrade them and desorb them. Such processes are suitable for cleaning surfaces and for activating polymer surfaces. The use of a plasma bears the distinct advantage of being able to penetrate inside complex structures. Generating plasma requires a specifically designed reactor. Generally, the specific gas to be used is injected to form an atmosphere at reduced pressure. Electromagnetic radiation (RF or microwave frequency) is then coupled into the enclosure, generating the plasma.
Some plasma systems operate at atmospheric pressure, removing the need for a vacuum-sealed enclosure. Plasma cleaning has an interesting feature, whereby its “effective” temperature (its kbT equivalent thermal energy) is of several thousand degrees. This is achieved by coupling energy into the gaseous phase without strongly increasing the substrate temperature. As an example, this process may clean carbon residue from surfaces without excessive heating. The plasma cleaning techniques also find application in the activation of other inorganic substrate surfaces such as metals and organic polymer surfaces.
A beneficial aspect of the present invention is that one or a combination of sub-atmospheric, atmospheric and super-atmospheric plasma-aided processes can be performed. This gives a range of beneficial plasma effects, for example producing supercritical ozone in high pressure plasmas produces a powerful in-situ cleaning agent.
Plasma is loosely defined as a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. It has been dubbed the “fourth state” of matter with its properties being similar to those of a gas and liquid.
Various types of plasmas may be created depending upon pressure, temperature, gas type, and frequency, among other facors. Moreover, low and high temperature plasmas are formed using either a low pressure or high pressure, respectively. High temperature plasma is found at atmospheric or super-atmospheric pressures between 1 atm and 100 atm or more with the beneficial cleaning effect due primarily to high energy oxygen radicals or ozone. Low temperature plasmas, used for surface modification and thin film organic cleaning, are ionized gases generated at pressures between 0.1 and 2 torr. Low temperature plasmas work within a vacuum chamber where atmospheric gases have been evacuated typically below 0.1 torr. These low pressures allow for a relatively long free path of accelerated electrons and ions. Since the ions and neutral particles are at or near ambient temperatures and the long free path of the electrons, which are at high temperature or electron-volt levels, have relatively few collisions with molecules at this pressure the reaction remains at low temperature. By contrast, atmospheric plasmas of nitrogen, oxygen and carbon dioxide tend to dominated by reactive neutral species such as O atoms, metastable O2, and some O3. High pressure plasma reactions tend to be dominated by ozone or supercritical ozone. It has been suggested that atmospheric plasmas are more similar to low-pressure plasmas.
The ionization of the gases is accomplished by applying an energy field using one of two frequencies regulated by the federal government:                Radio frequency (RF) frequency—13.56 MHZ        Microwave (MW) frequency—2.45 GHz.        
RF plasmas exhibit significantly higher levels of ultraviolet radiation (UV), which in part explains the higher concentrations of electronically charged particles than found in other plasma sources. RF plasmas have also been noted to be more homogeneous, a trait that is critical in treating irregularly shaped and overly large objects. MW source plasmas are generated downstream or in a secondary environment. Downstream is defined as the plasma generated in one chamber and drawn by a vacuum differential into the work area or another chamber. Though this can be advantageous for organic removal from ion sensitive components it also produces a less homogeneous process resulting in the compromising of uniformity across the work area. In surface modification the effective depth of the modification is tens of nanometers so the uniformity of the process becomes increasingly important, rendering MW source plasmas a less desirable choice. The voltage which must be exceeded to ignite a plasma is given by the following equation:Vb=B(P·d)/[ln[A(P·d)]—ln[ln(1+1/γse)]]where                Vb—breakdown voltage,        P—pressure,        d—gap distance,        γse—secondary emission coefficient of the cathode, and A and B are constants found experimentally.        
Also, the type of dielectric fluid influences Vb. When a plasma interacts with a substrate surface and contaminants thereon, four primary effects can occur:                1. Removal of organic materials;        2. Cross-linking via activated species of inert gases;        3. Ablation; and        4. Surface chemical restructuring.        
A major problem that prevents adequate adhesion is the presence of organic contamination on the surface. Contamination may exist in the form of residues, mold release agents, anti-oxidants, carbon residues or other organic compounds. Oxygen plasma is excellent for removing organics and is commonly used for this purpose.
Oxygen plasma causes a chemical reaction with surface contaminants resulting in their volatilization and removal from the plasma chamber. Care must be taken in selection of cleaning process parameters to ensure that organics are completely removed. It is possible to “surface modify” the contamination instead of removing it and thus still have a barrier layer which will cause adhesion to fail. Critical parameters may include sufficient power density to remove but not polymerize the organics or the addition of other gases to facilitate the prevention of polymerization. When exposed to the RF energy field, oxygen (O2) is broken down into free radicals at pressures above 0.1 torr is the most reactive element in the plasma and will readily combine with any organic hydrocarbon. The resultant combination is water vapor, CO and CO2 which is carried away in the vacuum or fluid stream. The reaction is by its nature complete with no residual surface products, however non-organics such as salts are not so readily removed. Sufficient RF energy must be applied to produce a high plasma density. Lower power densities not only remove contamination at a slower rate but also can actually impede the removal process. While the top layers of organic are being removed with low power density, underlying layers may cross-link in three dimensions creating a stable but un-removed new structure.
When generating plasma using inert noble gases such as helium or argon, the plasma will break C—C or C—H bonds by ion and UV bombardment. These free radicals in turn recombine on the surface causing a stable cross-linking of the surface structure. The improved bond strength of the surface can be a very desirable effect. In the case of PTFE (Teflon) treatment, it has been found that pretreatment with helium followed by the plasma of ammonia (NH3) will facilitate the bonding of a barrier layer to the PTFE, which in turn will be receptive to adhesion.
Etching of surfaces also can also be accomplished by plasma. Roughening of the surface can play a significant part in adhesion by increasing the total contact area between the adhesive and the subsurface. Etching is a result of gas selection or the length of time the surface is exposed to the plasma. Ablation can be accomplished with either active or inert gases and can be run to excess causing extremely porous surfaces by too long of an exposure to the plasma. The semiconductor manufacturing industry has used plasma etching as a primary treatment method for over 20 years. In addition the circuit board industry has used plasma as a means of etching polymers smeared in the drilling process. Hole smears prevent contact with plated through holes on multi-layer circuit boards. Smears are easily removed by plasma ablation regardless of how small the holes are.
Perhaps the plasma treatment effect offering the greatest potential is modification of the surface structure. By adding polar functional groups to the surface structure of the polymer we can greatly increase the surface energy and thus aid the adhesion to other substrate materials. Plasma treatment can be used to provide for the oxidation of the surface in much more uniform methods than by corona discharge or flame treatment. Large irregular surfaces also can be treated with little possibility of over-treatment, a drawback of both flame and corona methods. Elimination of adhesion primers or promoters, typically organic chemicals, is a benefit of using plasma prior to deposition of coatings.
Plasma reactions fall into two categories: chemical and mechanical. Chemical reactions result from a chemical interaction of the plasma with the surface of the product or contaminants attached to the surface. These reactions include oxidation and ablating the surface with such gases as oxygen, fluorine or chlorine. Mechanical reactions are generated with the use of noble gases such as argon or helium. Since these inert gases exist in their monatomic state the reaction is a kinetic energy transfer or, in simple terms a molecular scale sand blast. Dislodged contaminants can be swept away in the vacuum stream before they redeposit on the product or recombine on the product surface by selecting the proper process parameters. Inert gas plasma is also used to remove organic films and particulate matter from surfaces, which might readily oxidize such as silver or copper.
Conventional plasma systems have the following basic configuration:                1. A vacuum chamber for the reaction.        2. An energy source for gas ionization.        3. Control circuitry to regulate the time, gas flow and amount of energy.        4. A vacuum system to provide the low-pressure environment.        
Chambers are manufactured in either metal or glass depending on the application and the method of ionization. Quartz chambers are used in highly critical environments where sub-micron particulate generation is an issue. This includes the semiconductor, hybrid and medical analysis industries. For industrial applications, metal chambers are more prevalent and allow for the rougher handling environment accompanying that industry. Systems are even produced with tumbling chambers for surface modification of a large volume of small parts. Aluminum chambers offer an advantage over stainless steel chambers in that aluminum will develop a natural oxide layer that becomes a tough barrier to secondary reactions. Even the best stainless steel has been known to oxidize in a plasma environment and over time the oxidized surface can be a source of undesirable particulate.
Precision cleaning procedures using plasma are performed so that the cleaned surface will be coated or otherwise used as soon as possible after cleaning. Plasma surface treatment produces a molecular clean and water wettable surface with high surface energy. Thus, it has a tendency to adsorb particulate and organic contamination from the ambient environment. Storing the surfaces closed containers will reduce the adsorption of organic contaminant molecules from the ambient air. Particulate contamination will generally create unacceptable defects. Adsorbed organic contaminant molecules, generally less than full monolayer coverage, of order nm thickness, will generate a heterogeneous wettability. This may lead to non-uniform coatings, in particular if deposited from liquid media. Surfaces can never be perfectly clean in ambient air. It is generally recommended that the glass surface be used within a few minutes following its cleaning. As such, it is desirable to minimize the risk of coating defects caused by random ambient contamination.
As noted, plasmas will crosslink an organic contaminant. Crosslinking of thick films of organic contaminants such as oils will hinder cleaning action because it creates a barrier (i.e., carbon) to subsequent plasma reactions. As such, plasma cleaning is generally effective only when the surface is first gross cleaned to remove contaminant film down to 100 microns or less, for example by combining plasma with dense fluid cleaning—an important aspect of the present invention.
Conventional plasma processing thus described is more commonly called dielectric barrier discharge (DBD) plasmas which are of the variety comprising silent discharge, corona, transferred arc, plasma torch and low-pressure glow discharge. These plasmas can be created and used under various pressures, temperatures and using a variety of electrode configurations. Moreover, DBD plasma behavior is observed in solid, liquid and gas phases. Atmospheric pressure glow discharge (APGD) is getting attention today for various applications ranging from air pollution treatment, ozone production and more recently surface cleaning. However, high pressure plasma processing (i.e., P>1 atm) is still a relatively unexploited area. No known prior art exists for creating and using dense fluid plasmas (i.e., supercritical fluid plasmas, solid spray plasmas, liquid spray plasmas) taught herein.
The fundamental properties of DBD plasmas—for example the production of excited species, radicals, ozone and ultraviolet radiation—lead to photophysical and photochemical processes which readily lend themselves to the development of new enhanced surface cleaning, coating or modification processes, especially when used in combination with the variable solvent geometry and many other unique characteristics of dense fluids.
As such, novel and beneficial aspects of the present invention are derived from the combination of dense fluid cleaning solvents such as solid, liquid and supercritical carbon dioxide and application techniques with various types of DBD plasmas (i.e., using different fluid combinations, fluid pressures, temperatures, frequencies, voltages, DBD electrode configurations)—called dense fluid plasma herein. Dense fluid plasma technology enables new surface treatment possibilities not possible using either technology alone. The unique properties of dense fluids, for example being rather inert, having very low dielectric constants (i.e., k<3), and behaving as cleaning solvents allow them to be uniquely exploited herein as dielectric barrier discharge compounds, cleaning agents, extraction agents, drying agents, surface modification agents, deposition agents, plasma atmospheres, and anti-solvent agents within a full range of temperatures and pressures. Moreover, exemplary plasma-aided dense fluid processes taught herein are performed simultaneously or sequentially in a single process reactor which keeps the substrate surface clean, or un-compromised, in-between each process or operation.
Another novel aspect of the present invention is that the plasma process itself may be engineered to provide variable ion concentration—a selective plasma surface treatment. Since ion flux increases with decreasing pressure, adjusting pressure during various uses of plasma in the present invention is a way of controlling selectivity. This is very important in surface treatment applications where a substrate surface, for example a semiconductor wafer, contains both organic and inorganic structures. These structures interact with and react to different plasmas in different ways and at different reaction rates. For example, a less energetic plasma (i.e., atmospheric or super-atmospheric plasma) may be more suitable for an organic photoresist residue removal, when used with a dense fluid cleaning agent, because it decreases the chance for ion damage of adjacent inorganic or organic surface structures. Thus, having a selective plasma treatment is very beneficial—having a selective dense fluid plasma surface treatment technology is enabling.
Finally in another example the present aspect can uniquely produce a 3-dimensionally ultraclean sterile implantable medical device wherein its subsurface pores have been treated to remove as much as 8% by weight of unreacted monomer oils, during which its exposed surfaces are bathed in a sterilizing and surface energy modifying plasma—all in a single operation. This new and improved medical device exhibits enhanced performance such as increased tensile strength which increases its longevity under stress, extremely low potential for leaching of interstitial contaminants (i.e., silicone fluids) into body fluids, its water-wettable surfaces can be more readily adhesively bonded.
Spin Coating
Spin coating processes are used throughout many different industries including semiconductor wafer fabrication, optical glasses, and LCD manufacturing under clean room conditions and with fully automated handling. The coating thickness may vary between several hundreds of nanometers and up to 10 micrometers. Even with non-planer substrates very homogeneous coating thickness can be obtained. The quality of the coating depends on the rheological parameters of the coating liquid. Another important parameter is the Reynolds number of the surrounding atmosphere. If the rotation velocity is in a range, that the atmospheric friction leads to high Reynolds numbers (turbulences), disturbances in the coating quality are observed. The final thickness of a spin coated layer on the processing and materials parameters like angular velocity, viscosity and solvent evaporation rate by the semi-empirical formula;H=A·ω−B where                H—coating thickness,        ω—angular velocity; and        A and B are constants which are determined empirically.        
Spin coating and spin cleaning technology as practiced by the present inventor and others using dense fluids as noted herein suffer from a lack of control of the various surfaces such as atmosphere-dense fluid-substrate and other factors such as drag force, viscosity, fluid chemistry and surface chemistry—all of which are major components in optimizing and controlling the production of thin or thick films and coatings and removing undesirable substances from surfaces. The present invention teaches methods and processes which overcome these limitations.
Chemical Coating
Chemical coating is a process where a chemical reaction, e.g. the reduction of a metal is involved. The most common process is the fabrication of mirrors where the glass surface acts as a nucleating agent for the reduction of Ag+ to Ag0 in presence of reducing agent. The vast majority of all mirrors still are fabricated using this process. Another technology, which is suitable as an example for precipitating copper layers on glass, is the currently metalization process with commercially available liquids after seeding of the surface.
The present invention is teaches methods and processes for producing chemical coatings on substrate surfaces using a process of first coating a substrate with an un-reacted substance and then selectively reacting said first coating with a secondary agent such as plasma, hydrogen, and others to produce the desired chemical coating.
Patterning, Drying and Curing Techniques
Another aspect of the present invention is precision patterning, drying and curing of coated critical substrates such as IC, optical and MEMS wafers. Drying and curing techniques are important for obtaining the appropriate coating properties, as in a lithographic patterning or developing operations. The process may be made very selective through patterned laser exposure. Furthermore, organic polymer or organic-inorganic hybrid coating materials can be cured by a low temperature IR treatment or UV-curing, or reacted with a plasma. The present invention provides methods for selective removal of resists and resist solvents from patterned wafers as well as plasma etch clean-up techniques.
As line sizes becomes smaller and the complexity of semiconductor integrated circuits increases, it is clearly desirable to have a wet processing technique, including a method and apparatus, that actually removes unwanted organic films and particles, prevents additional particles, and does not introduce stains on the wafers. The complete cleaning technique may also include a step of gross drying the wafers, without other adverse results. A further desirable characteristic includes reducing or possibly eliminating the residual water absorbed on wafer surfaces and edges when the gross water phase is removed. The water left absorbed on such surfaces and edges often attracts and introduces more particles onto the semiconductor wafer and is a outgas and adhesion contamination in subsequent photoresist deposition following cleaning and drying operations. The aforementioned conventional techniques fail to provide such desired features, thereby reducing the die yield on the semiconductor following optical printing or lithographic processes.
There is a present need to provide an alternative and integrated surface preparation and deposition technique which overcomes the limitations of conventional technology described above. As such, the present invention relates to the fields of surface cleaning, surface modification, and deposition. The present invention is illustrated in several examples including surface cleaning, surface modification and deposition of an organic or metal-organic film or coating upon a substrate surface. The exemplary substrate surface is a wafer containing a surface mask film such as silica, polysilicon, silicon nitride, silicides, and metals. However it will be recognized that the invention has a wider range of applicability. Merely by way of example, the invention can also be applied to deposition of organic precursors, lubricants, bactericidal agents and other substances wherein a thin film is desired.
The present invention incorporates a unique isobaric dense fluid condensation shear cleaning process, a novel plasma-assisted drying process, plasma-based pre-cleaning and post-treatment steps, a dense fluid condensation shear deposition process, and post-cleaning and deposition reaction steps such as thermal treatment, plasma cleaning, and reactive gas treatments, among many other aspects. For example, the present invention teaches performing multi-step processing of dense fluid cleaning and plasma cleaning in a single process tool. In still another example, the present invention teaches a method, process and apparatus which embodies precision cleaning, drying, surface modification, and deposition within a closed controlled environment.